Prefoamed polyolefin beads produced by extrusion

ABSTRACT

Prefoamed polyolefin beads, produced by extrusion followed by granulation, have a crystal structure with a double peak.

The invention relates to prefoamed polyolefin beads produced byextrusion followed by granulation.

Prefoamed polyolefin beads, in particular those based on a copolymer ofpropylene with small amounts of ethylene, are increasingly employed forthe production of foam moldings in automotive construction, packaging,and in the leisure sector. The foam beads are usually produced bysuspending fine polyolefin granules in water, impregnating them with avolatile blowing agent at superatmospheric pressure and at temperaturesnear to their softening temperature, releasing the pressure and thusfoaming the polyolefin (see EP-A 53333). The polyolefin in the foambeads produced in this way has a specific crystal structure which, in aDSC curve, shows a double peak: a peak typical of the polyolefin and ahigh-temperature peak.

EP-A 588 321 describes a particularly elegant and simple process for theproduction of prefoamed polypropylene beads, in which polypropylene ismelted in an extruder, and a blowing agent is injected into this meltand homogeneously incorporated at from 125° to 250° C. The homogeneousmaterial is extruded through a round-section die, during which itexpands, and is chipped to give foam granules. The polypropylene whichis employed should have a melt strength of from 5 to 40 cN and abranched structure. However, it has been found that foam beads producedin this way have a very narrow processing range for production ofmoldings and do not fuse adequately with one another. For this reason,the moldings have relatively poor mechanical properties. In particular,the elongation at break and the tensile strength are unsatisfactory.

It is an object of the present invention to provide, starting fromconventional, ie. not melt-strengthened polyolefin, by extrusion andsubsequent granulation, prefoamed polyolefin beads which can be fusedwithout difficulty to give moldings with good mechanical properties.

We have found that this object is achieved if the polyolefin in the foambeads has a crystal structure with a double peak in which the distancebetween the two peaks is at least 5° C., preferably greater than 8° C.

For the purposes of the present invention, polyolefins are crystallineolefin polymers whose X-ray crystallinity at 25° C. is greater than 25%.Suitable polyolefins for the process are low-, medium- and high-densitypolyethylenes having a density of from 0.88 to 0.965 g/cm³,polypropylene, and ethylene and propylene copolymers comprising at least50 mol % of ethylene or propylene units respectively. Suitablecomonomers are, for example, α-alkenes with up to 12 carbon atoms, suchas ethylene, propylene, butene, pentene, hexene or octene, or vinylesters, such as vinyl acetate, or acrylates, methacrylates, maleates orfumarates of alcohols containing from 1 to 8 carbon atoms, copolymers ofethylene and acrylic acid or its derivatives, and ionomers. Mixtures ofdifferent olefin polymers can also be used.

Preference is given to ethylene-propylene copolymers and copolymers ofethylene and/or propylene with a C₄ - to C₈ -α-olefin.

Particularly preferred propylene copolymers are copolymers of propylenewith from 0.5 to 6% by weight of ethylene or from 0.5 to 15% by weightof 1-butene. Particularly preferred ethylene copolymers are copolymersof ethylene with from 1 to 18% by weight of 1-butene, 1-hexene or1-octene.

Particular preference is also given to copolymers of propylene with from0.5 to 6% by weight, preferably from 1.5 to 6% by weight, of ethyleneand from 0.5 to 6% by weight, preferably from 1.5 to 6% by weight of aC₄ - to C₈ -α-olefin, such as 1-butene, 1-pentene, 1-hexene, 1-hepteneor 1-octene. The copolymers should have a torsion modulus of from 100 to900, preferably from 150 to 750 N/mm².

The copolymers are prepared in a known manner by copolymerization of themonomers using transition-metal catalysts. The copolymers are, ingeneral, of linear construction, not crosslinked and notmelt-strengthened. The crystalline melting point, determined by DSC, isgenerally from 95° to 170° C. The enthalpy of fusion, determined by DSC,is generally from 30 to 130 J/g, preferably from 35 to 120 J/g.Preference is given to copolymers with a melt flow index MFI (230; 2.16)(according to DIN 53 735) of from 0.1 to 50, in particular from 0.5 to20 g/10 min!.

The novel foam beads are preferably produced by saturating polyolefingranules with a blowing agent at a low temperature in an extruder,homogenizing the material and holding it at elevated temperatures, andfinally extruding it with accompanying expansion.

A. In a first zone A, the polyolefin granules are saturated with avolatile blowing agent. It is possible to use a fine grit with particlesizes from 0.01 to 10 mm which arises directly from the polymerization,or granules with particle sizes from 0.1 to 10 mm which arise fromextrusion of the polyolefin. Suitable blowing agents are organiccompounds with boiling points of from 0° to 150° C., for examplen-butane, isobutane, n-pentane, isopentane, neopentane, cyclopentane orcyclohexane, heptane, octane, or halogenated hydrocarbons, and mixturesof these. The amount of blowing agent depends on the target bulk densityof the foam granules and is preferably from 5 to 40, in particular from8 to 30 parts by weight, based on 100 parts by weight of polyolefin.Conventional additives, such as dyes, pigments, nucleating agents,stabilizers, flame retardants, lubricants and antistats, can also beadded in efficacious amounts at this point.

The saturation of the polyolefin granules with the blowing agent ispreferably carried out in the feed zone of an extruder, where the shearforces are low. It is important that the initial swelling takes place ata relatively low temperature, ie. the temperature must be from 50° to180° C., preferably from 100° to 150° C., below the crystalline meltingpoint of the polyolefin and at least 10° C. below the boiling point ofthe blowing agent, so that this does not evaporate. By operating under apressure of up to 10 bar above atmospheric pressure, low-boiling blowingagents, such as butanes, can even be metered in at room temperature. Inthis case, the polyolefin granules are, for example, fed into zone A viaa rotary valve. In the case of the preferred polypropylene copolymerswith crystalline melting points of from 100° to 165° C., the temperaturein zone A is preferably from 10° to 40° C., in particular roomtemperature. The residence time in this zone is preferably from 0.1 to 5min. The polyolefin is saturated with the blowing agent to the extentthat its viscosity is sharply reduced.

B. This reduction in viscosity allows the homogenization in zone B to becarried out at from 2° to 50° C., preferably from 5° to 40° C., belowthe crystalline melting point of the pure polyolefin. This is a furtheradvantage in comparison with the process of EP-A 588 321, whereextrusion is carried out at significantly higher temperatures. Thishomogenization with simultaneous temperature increase is preferablycarried out in a second zone (homogenizing zone) of the extruder alreadyused in step A. Carbon dioxide or nitrogen can be metered into zone B insmall quantities (eg. from 0.1 to 5% by weight) for regulation of thecell diameter.

C. Finally, the material is held for some time, preferably from 1 to 60min, at from 2° to 50° C. below the crystalline melting point of thepolyolefin. This is expediently carried out in a holding zone C,arranged downstream of zone B, for example in a static mixer (Sulzermixer) or in a low-shear extruder (single-screw extruder).

For improved material transfer between zones B and C, particularly if astatic mixer is used, it may be advantageous to provide a gear pump atthe transition between these zones.

D. Finally, the material is extruded, expediently through at least oneround-section die, into the open atmosphere. The polyolefin, whichexpands under the sudden pressure reduction, is immediately granulated;granulation is expediently carried out by die-face cutting.

The resultant foam beads are ellipsoidal to spherical. The mean diameteris from 2 to 20 mm. Their bulk density can be adjusted within widelimits of from 0.01 to 0.2 g/cm⁻¹, preferably from 0.01 to 0.15 g/cm⁻¹.The cell count is generally from 0.1 to 1000 cells per mm².

Moldings with good mechanical properties can be produced by fusion ofthe novel prefoamed beads in a conventional manner.

EXAMPLE

15 kg/h of a particulate copolymer of propylene with 2.5% by weight ofethylene (crystalline melting point 147.5° C.) and 0.15 kg/h of talcwere fed continuously into the feed zone A of a twin-screw extruder(D=60 mm, L=36 D). The blowing agent (3 kg/h of cyclopentane) wasinjectedbetween the feed zone and the transition point into thehomogenizing zone. The temperature at this point was 25° C., and thepressure was atmospheric pressure. There then followed the transitioninto the homogenizing zone B, where the extruder was heated to a barreltemperatureof 100° C. This heating, together with frictional heat,increased the material temperature to 125° C., whereupon the materialimmediately melted. Following this, a pressure of about 170 bar wasdeveloped by a gear pump which was maintained at 130° C. The gear pumpwas followed by a holding zone C, consisting of 4 Sulzer mixers (DN=80mm). These were maintained at 130° C. After a holding time of 45 min,the polymer containing blowing agent was extruded through a 2 mmdiameter round-section die and granulated using a Berstorffgranulator.The foam beads obtained had a diameter of 10 mm. Their bulkdensity was 0.22 g/l. The DSC curve shows two maxima at 146° C. and160°C.

We claim:
 1. A process for producing prefoamed polyolefin beads having acrystal structure which in a DSC curve (obtained by heating from 3 to 6mg of the foam beads to 220° C. at a heating rate of 20° C./min in adifferential calorimeter), show a peak typical of the polyolefin and ahigh-temperature peak where the distance between the two peaks is atleast 5° C., which comprises:A. saturating granules of a crystallinepolyolefin with a physical blowing agent at a temperature which is from50° to 180° C. below the crystalline melting point of the polyolefin andat least 10° C. below the boiling point of the blowing agent, in a feedzone of an extruder, B. homogenizing the polyolefin containing blowingagent at increasing temperature in a homogenizing zone of the extruder,where the material temperature is from 2° to 50° C. below thecrystalline melting point of the pure polyolefin, C. holding the melt atthis temperature for from 1 to 60 min at this temperature in a holdingzone, and D. extruding the polyolefin containing blowing agent into theopen atmosphere, with accompanying expansion, and finally granulatingthe material.
 2. The process as claimed in claim 1, wherein thepolyolefin is a random copolymer of propylene with from 0.5 to 6% byweight of ethylene or from 0.5 to 15% by weight of 1-butene.
 3. Theprocess as claimed in claim 1, wherein the prefoamed polyolefin beadshave a bulk density of from 0.01 to 0.2 g/cm.
 4. The process as claimedin claim 1, wherein the prefoamed polyolefin beads have a diameter offrom 2 to 20 mm.